1. Field of the Invention
This invention relates to an improved process for the preparation of multilayer printed circuit boards and is more particularly concerned with improving the adhesion between copper circuit patterns and polymeric substrates.
2. Description of the Prior Art
Multilayer printed circuit boards are assemblies of several two-sided printed circuit boards bonded to each other through layers of semi-cured polymeric material by lamination using heat and pressure. The multilayer boards provide high packaging density, short conductor lengths and good reliability. At the same time weight and space are conserved. Accordingly, such boards are finding increased use.
The two-sided printed circuit boards from which the multilayer printed circuit boards are built are generally fabricated from a polymeric substrate such as phenolic resins, epoxy resins, polyimides, polyesters and the like, clad on both sides with a thin layer of copper. Printed circuits are formed on the copper layers by conventional techniques. Illustratively, a layer of photoresist is coated over the copper layer, exposed imagewise and developed to yield a relief resist image on the copper layer. The exposed copper is etched away and the resist image is removed leaving the copper circuit pattern exposed. The latter is then chemically treated to form thereon a surface layer of copper oxide, tin or other adhesion promotor as described in, for example, U.S. Pat. Nos. 2,955,974; 3,177,103; 3,198,672; 3,240,662; 3,374,129; 3,481,777; 4,409,037 and 4,512,818.
Any desired number of the two-sided boards so prepared is then assembled into the multilayer circuit board by forming a stack of the boards with layers of semi-cured polymeric non-conductive material such as epoxy resin interposed between the individual boards. The assembly is laminated by application of heat and pressure. Typical of such laminating conditions are pressing at about 300 psi to about 400 psi at a temperature at about 150.degree. C. to about 205.degree. C. for a time of up to about 4 hours. The circuits on the inner and outer layers are then connected by drilling an array of holes in the board, cleaning the holes to remove resin smear by treatment with acids and the like and thereafter plating with copper to render the sides of the holes conductive.
The use of a copper oxide layer as an adhesion promoter between the printed copper circuit patterns and the intermediate polymeric dielectric layers of the multilayer board has been employed for a number of years. However, problems have arisen which have prompted a search for improved coatings. Thus total or local delamination of copper circuit pattern layers and polymeric layers can occur due to poor adhesion and/or the susceptibility of the copper oxide layer to dissolve when exposed to acidic and/or reducing solutions in later treatment of the multilayer board.
A major problem due to a combination of poor adhesion and the susceptibility of the copper oxide to dissolution is that known as the "pink ring" or "halo" effect. When the multilayer boards are drilled in preparation for making interlayer connections by plated through holes, a combination of stress and heat at the drill interface can cause local delamination between the copper circuit pattern inner layers and the intermediate polymeric resin layers. The copper oxide coating which is then exposed may be dissolved by the acid or like solutions employed to clean the resin smear from the drilled hole prior to plating the hole. Removal of the copper oxide coating around the edges of the copper circuit pattern adjacent to the through-hole reveals the underlying copper as a pink ring or halo. This causes cosmetic as well as functional problems.
A number of approaches to solving this widespread problem have been tried. Thus, pressing conditions employed in the lamination have been varied to optimize the adhesion promoted by the copper oxide coating. This has met with limited success. Thicker coatings of copper oxide have been used in order to retard the dissolution process by shear volume of the copper oxide present. However, the thicker coatings give poor adhesion and therefore do not solve the problem. Another approach has been to use a coating of tin in place of the copper oxide layer as an adhesion promoter but the level of adhesion achieved has not been sufficient to overcome the problem.
It has now been found that the problems which have been encountered hitherto due to delamination, either total or local, of the copper circuit patterns and the intermediate polymeric resin layers in multilayer printed circuit boards can be overcome by a post treatment of the adhesion promotor layer of copper oxide, tin or like promoters, prior to lamination of the individual two-sided boards into the multilayer board.